As for working to bend a pipe, various kinds of working such as press bending, compression bending, extract bending, draw bending and the like are known heretofore, among which a rotary draw bending is most popular. According to an ordinary rotary draw bending, a pipe is held by a clamp die against a bend die with a groove formed on its outer peripheral surface, and the bend die and clamp die are rotated, with the pipe being pressed toward the bend die by means of a pressure die, then the pipe is moved in a tangential direction, thereby to be bent along the groove of the bend die, as disclosed in the second columns of Patent document 1, for example, and also disclosed in Patent document 2, in its paragraphs (0003)-(0006) and FIG. 11, wherein the bend die is described as a roll die.
In Patent document 2, with respect to a wiper or shoe provided for preventing a crinkling from being created on the inner side of a bent portion of the pipe, a specific wiper is proposed to do with wear or breakage caused by a sliding motion, as described in its paragraphs (0013) and (0014). Likewise, in Patent document 3, it is described in its paragraph (0005) as an object to provide a pipe bending apparatus having a wear resistance, being used for various kinds of pipes without causing a problem, and having a very long life without requiring adjusting operations very often, and such a pipe bending apparatus is proposed in its paragraph (0006) that is characterized in that the pipe bending apparatus has a bend die with its outer peripheral surface formed in a circular arc of a predetermined curvature to bend a pipe, a clamp member clamping the pipe with the bend die, and a wiper rotating the clamp member about the bend die to prevent a crinkling from being created when the pipe is bent, and that a tip end portion of the wiper in the rotating direction of the clamp member is extended along the curvature of the outer peripheral surface of the bend die beyond an initial point for bending the pipe.
Furthermore, in Patent document 4, with respect to a method and apparatus for quickly and accurately changing die sets for different-sized tubing to be bent or for different types of tube bending operations, it is described in its page 7 that a preassembled die set has been devised for tube bending apparatus wherein the die set is comprised of a bend die, clamp die and pressure die adapted to be mounted on a spindle of a tube bending table, the improvement comprising first means releasably interconnecting the pressure die and clamp die to the bend die in predetermined, aligned relation to one another and to the bend die, and handling means for engaging the die set in order to simultaneously lift and remove said dies comprising each die set from the table. And, it is described in its page 8 that many tube bending operations require the use of a wiper die and mandrel, which may also provide a part of each preassembled die set when needed, and such an embodiment that the wiper die is joined to the bend die by a wiper die arm is disclosed in its page 15 and FIG. 6.